RHINEURA 49 



GENERAL ACCOUNT OF THE EYE OF RHINEURA. 



The eye of Rhineura Jiorldana is not visible externally, nor is there any indica- 

 tion where it formerly came to the surface. The side of the head is continuously 

 covered with plates. There are 4 labials (i, 2, 3, and 4, of fig. 17), the posterior 

 of which is comparatively large. Above the labials from in front backward lie a 

 single nasal (5), a single loreal (6), a single preorbital (7), and a group of temporals 

 (8). Above this series of plates lie a supranasal (9), joined to its fellow of the other 

 side, a prefrontal (n),and 2 supraciliaries (12, 13). In heads cleared with xylol 

 the black eye can be seen to lie underneath the angle between the 2 supraciliaries 

 and the preorbital. 



The dermis and epidermis over the eye are not different from these structures 

 over neighboring regions except that in one instance (plate 3 c, df) a solid column of 

 cells 32 /A thick extends from Harder 's gland to near the epidermis, without, how- 

 ever, fusing with the latter. Fisher found that in Trogonophis the epidermis is 

 reduced to half its thickness and free from pigment over the eye. In Amphisbcena 

 strauchi and A. darwini the skin is not thinner and the pigment is little or not at 

 ah 1 less over the eye. A conjunctival sac has been described for various Amphis- 

 baenians. No such structure is present in Rhineura. 



Harder's gland (plate 3, figs. B, c, H.gl.} is out of all proportion to the size of 

 the eye. In a horizontal section it measures about 4 times as long as the eye 

 (medio-laterally) and 3 times as wide (antero-posteriorly). Duvernoy found that 

 in Typhlops Harder's gland is 10 times as great as the eye. It is divided into 2 

 distinct lobes, that over the anterior face of the eye is histologically quite different 

 from that over the posterior face. In vertical section the gland is seen to entirely 

 surround the eye except sometimes at its lower posterior quarter. The large size 

 of Harder's gland has given rise (Duvernoy) to the conclusion that its function is 

 not connected with the eye. Its secretion is poured directly into the tear duct and 

 through it into the nasal cavity. 



The distance of the eye beneath the outer surface of the epidermis measures 

 between 320 and 560 p. in specimens between 280 and 310 mm. long. It is sur- 

 rounded by 2 layers of connective tissue. These are thin over the distal half of the 

 eye. Over the proximal narrow end of the eye they become thick ; and since they 

 are prolonged beyond the eye, stain a different tint, and readily become separated, 

 they are easily distinguishable. They probably represent the sclera and choroid. 

 If so, the choroid is practically free from pigment except possibly in rare instances 

 where a few pigment granules were detected in cells closely applied to the eye. 

 There is no indication of any differentiation into a cornea or capsule of any sort. 

 The fibrous sheaths are at the proximal end drawn out into a cone. A supposed 

 scleral cartilage has been found in one individual. Here a bar about 20 p- thick 

 extends from over the center of the distal face of the eye for 160 p. around its pos- 

 terior face. It stains and has the structure of bone rather than of cartilage. No 

 traces of any muscles have been found connected with the eye. 



The eye is directed outward and forward. Its axis is horizontal and makes an 

 angle of about 60 with the sagittal plane of the body. It does not occupy a defi- 

 nitely fixed position on its axis, for in the eye of one side the choroid fissure was found 

 directed caudad, in the other eye ventrad. It is irregularly pear-shaped, with its 

 anterior face convex, its posterior face flat or even concave. The eyes in 3 speci- 

 mens give the following measurements in microns : 



